Verkkodumbledore knew the first part of the unbreakable vow between snape and narcissa because harry overheard snape and draco's conversation.
Verkkowhy did snape make the unbreakable vow?
Ostensibly, promising to blow his cover and kill dumbledore if need be might go against the wishes of voldemort.
Almost all of the preparation of the vow is.
Verkkohe did it to eliminate the threat of bellatrix's suspicions.
Bellatrix was a powerful member of the death eaters, having her doubt his loyalty could have destroyed his.
Verkkowhen harry overhears snape questioning draco regarding his secret activities, snape appears uncertain about the details of draco's mission, and draco.
Verkkoafter some general mingling at the party, draco malfoy is caught prowling the corridors and professor snape takes him away to an empty classroom,.
Verkkoit goes a bit deeper, snape did it knowing full well it would get back to voldemort.
Snape already planned on killing dumbledore and agreed with dumbledore to do it.